Methods of isomerizing xylenes with a catalyst reduced in the presence of hydrogen and a base

ABSTRACT

A xylene isomerization process includes introducing gas comprising hydrogen and a base to a reaction zone in which a catalyst comprising a Group VIII metal and a zeolite support resides. In one embodiment, the base may be formed in situ within the reaction zone from nitrogen and hydrogen that are introduced to the reaction zone. In another embodiment, the base is introduced directly to the reaction zone. The conditions in the reaction zone are effective to reduce the catalyst. A stream comprising C 8  aromatics, e.g., xylenes and ethylbenzene may then be fed to the reaction zone containing the reduced catalyst. The reaction zone may be operated at conditions effective to isomerize the xylenes and hydrodealkylate the ethylbenzene. The xylene loss during the isomerization of the xylenes is lowered as a result of using the catalyst reduced in the presence of the gas comprising a base and hydrogen.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

FIELD OF THE INVENTION

The present invention generally relates to xylene isomerizationcatalysts and methods of making and using same, and more particularly toisomerization of xylenes with a catalyst that has been reduced in thepresence of hydrogen and a base.

BACKGROUND OF THE INVENTION

The xylene isomers orthoxylene, metaxylene, and paraxylene, areimportant chemical intermediates. Orthoxylene may be oxidized to makephthalic anhydride, which is used to make phthalate-based plasticizersamong other things. Metaxylene may be oxidized to make isophthalic acid,which is used in unsaturated polyester resins. Paraxylene may beoxidized to make terephthalic acid, which in turn is used to makepolymers such as polytrimethyleneterephthalate,polybutyleneterephthalate (PBT), and polyethyleneterephthalate (PET).PET is one of the largest volume polymers in the world and is used tomake PET plastics (e.g., two liter PET bottles). It is also used to makepolyester fiber, which in turn is used to make clothes and otherfabrics. Given the large market for PET plastics and fibers, there is asubstantial demand for high purity paraxylene, which is several timeslarger than the demand for orthoxylene and metaxylene. To help meet suchdemand, orthoxylene and metaxylene may be isomerized to paraxylene viause of an isomerization catalyst.

Paraxylene may be produced by reforming or aromatizing a wide boilingrange naphtha in a reformer, for example, a Continuous CatalyticReformer (CCR) or semi-regenerative reformer, followed by distillationof the naphtha reformer effluent into a C₈ aromatics fraction(containing aromatics having eight carbon atoms). This C₈ aromaticsfraction comprises near equilibrium amounts of orthoxylene, metaxylene,and paraxylene along with ethylbenzene. The paraxylene is separated fromthe other components in this C₈ aromatics fraction in a separation uniteither by a crystallization process or by an adsorption process, therebyforming a paraxylene-depleted stream. The paraxylene-depleted stream maybe further processed by passing it over a xylene isomerization catalystin a xylene isomerization unit, wherein orthoxylene and metaxylene areisomerized to paraxylene.

Xylene isomerization catalysts may comprise a ZSM-5 zeolite support.However, these catalysts have some unwanted side reactions that consumethe xylene isomers and reduce the overall xylene selectivity. Such sidereactions are particularly a problem when the catalyst is of the HZSM-5type, wherein H refers to the ZSM-5 being predominately in the hydrogenform. The HZSM-5 catalyst has several acid sites that promote unwantedcracking reactions, resulting in a relatively high amount of xylene lossand thus a decrease in the production of paraxylene. A need thereforeexists to reduce the xylene losses that occur during the xyleneisomerization process such that the overall xylene selectivity and thusthe production of paraxylene is increased.

SUMMARY OF THE INVENTION

Methods of isomerizing xylenes include introducing hydrogen and a baseto a reaction zone in which a catalyst comprising a Group VIII metal anda zeolite support resides. The conditions in the reaction zone aredesirably effective to reduce the catalyst in the presence of the baseand the hydrogen. In one embodiment, the base may be formed in situwithin the reaction zone from nitrogen (N₂) and hydrogen (H₂) that areintroduced to the reaction zone. In another embodiment, the base isintroduced directly to the reaction zone and may comprise, for example,ammonia (NH₃), alkyl amines, hydrazine, or combinations thereof. Astream comprising C₈ aromatics, e.g., xylenes and ethylbenzene, may thenbe fed to the reaction zone containing the reduced catalyst. Thereaction zone may be operated at conditions effective to isomerize thexylenes and hydrodealkylate the ethylbenzene. Reducing the catalyst inthe presence of the hydrogen and the base decreases the xylene lossduring the isomerization of the xylenes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Disclosed herein are xylene isomerization catalysts comprising a HZSM-5zeolite support, a SSZ-57 zeolite support, or both and one or more GroupVIII metals. The xylene isomerization catalysts may be activated byreduction in the presence of hydrogen and a base. The reduced catalystsmay be used for the isomerization of xylenes.

The xylene isomerization catalyst may comprise a HZSM-5 zeolite support.HZSM-5 zeolite refers to ZSM-5 zeolite that is predominantly in thehydrogen form, meaning that the ZSM-5 zeolite is in an acidic form asopposed to a basic form. A basic form is one where the ZSM-5 zeolite hassubstantial amounts of the original sodium, i.e., the sodium that ispresent in the as-synthesized ZSM-5 zeolite. The HZSM-5 zeolite may beformed by replacing most of the original cations of the ZSM-5 zeolitewith hydrogen using methods known in the art such as subjecting theZSM-5 zeolite to an ion exchange with ammonium salts followed bycalcination. Examples of suitable salts include but are not limited tochlorides, nitrates, sulfates, acetates, carbonates and combinationsthereof. The HZSM-5 zeolite may have greater than or equal to about 80%of the sodium ions replaced by hydrogen ions, alternatively greater thanor equal to about 90%, alternatively greater than or equal to about 95%,or alternatively greater than or equal to about 98%. In accordance withthese replacements, the amount of sodium remaining in the HZSM-5 zeolitewill depend on the original amount present, which in turn will depend onfactors such as the silica to alumina ratio. Keeping thesequalifications in mind, the amount of sodium left in the HZSM-5 zeoliteafter it has been converted to the hydrogen form may be less than about0.1%, alternatively less than about 0.06%, or alternatively less thanabout 0.03%, all percentages being by weight of the original sodium.

The HZSM-5 zeolite may be employed in a “bound” form, i.e., with arefractory oxide as a binder for the overall catalyst particle. Thebinder serves to hold the crystalline zeolite particles together in acatalyst particle of suitable size and suitable attrition resistanceupon handling and use in the isomerization process. Examples of suitablerefractory oxide binders include alumina, silica, titania, clay, ormixtures thereof. A suitable commercially available binder is CATAPAL®-Bbinder, which may be purchased from Sasol. The amount of binder used maybe in a range of from about 5% to about 65%, alternatively from about10% to about 50%, all percentages being by weight of the HZSM-5 zeolite.

The HZSM-5 zeolite may have a relatively small crystal size. Inembodiments, the crystal size of the HZSM-5 zeolite is less than about1.0 micron, alternatively less than about 0.9 micron, alternatively in arange of from about 0.2 micron to about 0.9 micron, or alternatively ina range of from about 0.2 micron to about 0.8 micron. The HZSM-5 zeolitemay have pore sizes or diameters in the range of from about 5 Angstromsto about 7 Angstroms, alternatively about 5.5 Angstroms. Zeolites withthese pore sizes are commonly referred to as “intermediate pore sizezeolites”, and are in contrast to the larger pore size zeolites such asfaujasite or the smaller pore size zeolites such Linde Type A anderionite. The structure of HZSM-5 zeolites is described by Kokotailo etal. in Nature, Vol. 272, Mar. 30, 1978, page 437, which is incorporatedby reference herein in its entirety. The pore size of the crystallinezeolite is delineated by the atomic structure. However, the pore sizemay be modified by components added to the HZSM-5 zeolite.

The HZSM-5 zeolite can be prepared in various manners. Suitablepreparation procedures are described in U.S. Pat. No. 3,702,886, whichis incorporated by reference herein in its entirety. In an embodiment,the HZSM-5 zeolite can be made by preparing a solution containing water,tetrapropyl ammonium hydroxide and the elements of sodium oxide, anoxide of aluminum or gallium, and an oxide of silica such that thesolution has the following composition in terms of mole ratios ofoxides:

TABLE 1 Broad Intermediate Narrow Range Range Range OH⁻/SiO₂ 0.07-1.0 0.1-0.8 0.2-0.75 R₄N+/(R₄N⁺ + Na⁺)  0.2-0.95 0.3-0.9 0.4-0.9  H₂O/OH⁻ 10-300  10-300 10-300 wherein R is propyl.The broad, intermediate, and narrow ranges are different embodiments ofthe mole ratios. In embodiments, the silica/alumina ratio of the ZSM-5zeolite is in a range of from about 10 to about 300, from about 30 toabout 200, from about 30 to about 150, from about 50 to about 100, andfrom about 70 to about 90. The mixture may be maintained at reactionconditions until the crystals of the zeolite are formed. Thereafter, thecrystals may be separated from the liquid and recovered. Typicalreaction conditions include a temperature in a range of from about 160°F. to about 400° F. for a period of from about 2 days to about 60 days,or alternatively a temperature in a range of from about 190° F. to about235° F. for a period of from about 7 days to about 21 days. The solidproduct may be separated from the reaction medium by cooling it to roomtemperature, filtering it, and washing it with water. The as-synthesizedZSM-5 zeolite may be further converted from a basic to an acidic form,as described previously, to provide the HZSM-5 zeolite that may be usedas a support for one or more catalytic metals selected from the GroupVIII metals of the periodic table.

The xylene isomerization catalyst may comprise a SSZ-57 zeolite support.SSZ-57 refers to a zeolite that can be prepared as described herein inboth the borosilicate or aluminosilicate phase. The term “borosilicate”refers to a zeolite containing oxides of both boron and silicon. Theterm “aluminosilicate” refers to a zeolite containing oxides of bothaluminum and silicon. In preparing SSZ-57 zeolites, aN-cyclohexyl-N-butylpyrrolidinium ammonium cation,N-propyl-N-cycloheptylpyrrolidinium cation orN-butyl-N-cyclooctylpyrrolidinium may be used as a templating agent. Ingeneral, SSZ-57 may be prepared by contacting an active source of anoxide selected from silicon oxide, germanium oxide and mixtures thereofand boron oxide or a combination of boron oxide and aluminum oxide,gallium oxide, indium oxide, titanium oxide or a mixture thereof withthe templating agent. In an embodiment, SSZ-57 may be prepared from areaction mixture comprising the following mole ratios of reagents: YOWaO_(b) from about 20 to ∞, OH⁻/YO₂ from about 0.1 to about 0.5, Q/YO₂from about 0.05 to about 0.5, M_(2/n)YO₂ from about 0.02 to about 0.4and H₂O/YO₂ from about 25 to about 80 wherein Y is silicon, germanium ora mixture thereof; W is boron or a combination of boron and aluminum,gallium, indium, titanium or a mixture thereof; M is an alkali metalcation, alkaline earth metal cation or mixtures thereof; n is thevalence of M (i.e., 1 or 2); a is 1 or 2; b is 2 when a is 1 (i.e., W istetravalent) or b is 3 when a is 2 (i.e., W is trivalent); and Qcomprises a N-butyl-N-cyclohexylpyrrolidinium cation,N-propyl-N-cycloheptylpyrrolidinium cation orN-butyl-N-cyclooctylpyrrolidinium cation. Alternatively, SSZ-57 may beprepared from a reaction mixture comprising the following mole ratios ofreagents: YO₂/WaO_(b) from about 35 to 90, OH⁻/YO₂ from about 0.2 toabout 0.3, Q/YO₂ from about 0.1 to about 0.2, M_(2/n)/YO₂ from about 0.1to about 0.25 and H₂O/YO₂ from about 30 to about 50.

In practice, SSZ-57 is prepared by a process comprising: (a) preparingan aqueous solution containing sources of at least one oxide capable offorming a crystalline molecular sieve and theN-butyl-N-cyclohexylpyrrolidinium cation,N-propyl-N-cycloheptylpyrrolidinium cation orN-butyl-N-cyclooctylpyrrolidinium cation in the presence of an anioniccounterion which is not detrimental to the formation of SSZ-57; (b)maintaining the aqueous solution under conditions sufficient to formcrystals of SSZ-57; and (c) recovering the crystals of SSZ-57.Accordingly, SSZ-57 may comprise the crystalline material and thetemplating agent in combination with metallic and non-metallic oxidesbonded in tetrahedral coordination through shared oxygen atoms to form across-linked three dimensional crystal structure. The metallic andnon-metallic oxides comprise an oxide selected from silicon oxide,germanium oxide and mixtures thereof and boron oxide or a combination ofboron oxide and aluminum oxide, gallium oxide, indium oxide, titaniumoxide or a mixture thereof. Typical sources of silicon oxide includesilicates, silica hydrogel, silicic acid, fumed silica, colloidalsilica, tetra-alkyl orthosilicates, and silica hydroxides. Boron, aswell as aluminum, gallium, germanium, titanium, and indium can be addedin forms corresponding to their silicon counterparts.

A source zeolite reagent may provide a source of boron. In most cases,the source zeolite also provides a source of silica. The source zeolitein its dealuminated or deboronated form may also be used as a source ofsilica, with additional silicon added using, for example, theconventional sources listed above. Use of a source zeolite reagent as asource of alumina for the preparation of a SSZ-57 zeolite support ismore completely described in U.S. Pat. No. 5,225,179, issued Jul. 6,1993 to Nakagawa entitled “Method of Making Molecular Sieves,” thedisclosure of which is incorporated herein by reference.

An alkali metal hydroxide and/or an alkaline earth metal hydroxide, suchas the hydroxide of sodium, potassium, lithium, cesium, rubidium,calcium, and magnesium, may be used in the reaction mixture; however,this component can be omitted so long as the equivalent basicity ismaintained. The templating agent may be used to provide hydroxide ion.Thus, it may be beneficial to ion exchange, for example, the halide forhydroxide ion, thereby reducing or eliminating the alkali metalhydroxide quantity required. The alkali metal cation or alkaline earthcation may be part of the as-synthesized crystalline oxide material, inorder to balance valence electron charges therein.

In an embodiment, the reaction mixture is maintained at an elevatedtemperature until the crystals of the SSZ-57 zeolite are formed. Thehydrothermal crystallization may be conducted under autogenous pressure,at a temperature between 100° C. and 200° C., alternatively between 135°C. and 160° C. The crystallization period may be greater than about 1day, alternatively from about 3 days to about 20 days. In an embodiment,the zeolite is prepared using mild stirring or agitation. During thehydrothermal crystallization step, the SSZ-57 crystals can be allowed tonucleate spontaneously from the reaction mixture. The use of SSZ-57crystals as seed material can be advantageous in decreasing the timenecessary for complete crystallization to occur. In addition, seedingcan lead to an increased purity of the product obtained by promoting thenucleation and/or formation of SSZ-57 over any undesired phases. Whenused as seeds, SSZ-57 crystals are added in an amount between 0.1 and10% of the weight of silica used in the reaction mixture. Once thezeolite crystals have formed, the solid product may be separated fromthe reaction mixture by standard mechanical separation techniques suchas filtration. The crystals may then be water-washed and dried, e.g., at90° C. to 150° C. for from about 8 to about 24 hours, to obtain theas-synthesized SSZ-57 zeolite crystals. The drying step can be performedat atmospheric pressure or under vacuum.

SSZ-57, prepared as described herein, has a mole ratio of an oxideselected from silicon oxide, germanium oxide and mixtures thereof toboron oxide or a combination of boron oxide and aluminum oxide, galliumoxide, indium oxide, titanium oxide or a mixture thereof greater thanabout 20; and has the X-ray diffraction lines of Table 2. SSZ-57 furtherhas a composition, as-synthesized and in the anhydrous state, in termsof mole ratios, shown in Table 3.

TABLE 2 As synthesized SSZ-57 Two Theta (deg)^(a) d-spacing IntensityI/I_(o) ^(b) 7.7 11.5 S 8.8 10.0 M 14.65 6.04 W 15.55 5.69 W 17.65 5.02W 20.85 4.26 W 23.05 3.86 VS 24.35 3.65 M 26.6 3.35 W 30.2 2.96 W 45.12.10 W ^(a)±0.15 ^(b)The X-ray patterns provided are based on a relativeintensity scale in which the strongest line in the X-ray pattern isassigned a value of 100. W(weak) is less than 20; M(medium) is between20 and 40; S(strong) is between 40 and 60; VS(very strong) is greaterthan 60.

TABLE 3 As synthesized SSZ-57 YO₂/W_(c)O_(d) 20-∞ M_(2/n)/YO₂ 0.01-0.03Q/YO₂ 0.02-0.05 where Y, W, c, d, M and Q are as defined previously.

SSZ-57 can be made essentially aluminum free, i.e., having a silica toalumina mole ratio approaching infinity. A method of increasing the moleratio of silica to alumina is by using standard acid leaching orchelating treatments. However, essentially aluminum-free SSZ-57 can besynthesized directly using essentially aluminum-free silicon and boronsources. SSZ-57 is generally prepared directly as a borosilicate. Lowersilica to alumina ratios may also be obtained by using methods whichinsert aluminum into the crystalline framework. For example, aluminuminsertion may occur by thermal treatment of the zeolite in combinationwith an alumina binder or dissolved source of alumina. Such proceduresare described in U.S. Pat. No. 4,559,315, issued on Dec. 17, 1985 toChang et al., incorporated by reference herein in its entirety. It isbelieved that SSZ-57 is comprised of a new framework structure ortopology which is characterized by its X-ray diffraction pattern. SSZ-57zeolites, as-synthesized, have a crystalline structure whose X-raypowder diffraction pattern exhibit the characteristic lines shown inTable 2 and is thereby distinguishable from other zeolites.

Crystalline SSZ-57 can be used as-synthesized, but may be thermallytreated (calcined). Usually, it is desirable to remove the alkali metalcation by ion exchange and replace it with hydrogen, ammonium, or anydesired metal ion. The zeolite can be leached with chelating agents,e.g., EDTA or dilute acid solutions, to increase the silica to aluminamole ratio. The zeolite can also be steamed; steaming helps stabilizethe crystalline lattice to attack from acids.

SSZ-57 can be formed into a wide variety of physical shapes. Generallyspeaking, the zeolite can be in the form of a powder, a granule, or amolded product, such as extrudate having a particle size sufficient topass through a 2-mesh (Tyler) screen and be retained on a 400-mesh(Tyler) screen. In cases where the catalyst is molded, such as byextrusion with an organic binder, the aluminosilicate can be extrudedbefore drying, or dried or partially dried and then extruded. SSZ-57 canbe composited with other materials resistant to the temperatures andother conditions employed in organic conversion processes. Such matrixmaterials include active and inactive materials and synthetic ornaturally occurring zeolites as well as inorganic materials such asclays, silica and metal oxides. Examples of such materials and themanner in which they can be used are disclosed in U.S. Pat. No.4,910,006, issued May 20, 1990 to Zones et al., and U.S. Pat. No.5,316,753, issued May 31, 1994 to Nakagawa, both of which areincorporated by reference herein in their entirety. Methods of preparingand utilizing SSZ-57 catalysts have been disclosed in U.S. Pat. Nos.6,544,495 and 6,616,830, both of which are incorporated by referenceherein.

The xylene isomerization catalyst may further comprise one or more GroupVIII metals such as platinum (Pt), palladium (Pd), nickel (Ni), cobalt(Co), rhodium (Rh), iridium (Tr), iron (Fe), ruthenium (Ru), osmium(Os), or combinations thereof. Alternatively, the catalyst may compriseplatinum, palladium, nickel or combinations thereof. The metal isbelieved to act as a hydrogenation/dehydrogenation component. In anembodiment in which the Group VIII metal is Pt, the amount of Pt presenton the catalyst may be in a range of from about 0.05% to about 1.0%,alternatively from about 0.05% to about 0.75%, or alternatively fromabout 0.075% to about 0.5%, all percentages being by total weight of thecatalyst. In an embodiment in which the Group VIII metal is Pd, theamount of Pd present in the catalyst may be in a range of from 0.1% to2.0%, alternatively from about 0.1% to about 1.5%, or alternatively fromabout 0.15% to about 1.0%, all percentages being by total weight of thecatalyst. In an embodiment in which the Group VIII metal is Ni, theamount of Ni present in the catalyst may be in a range of from about0.1% to about 20%, alternatively from about 0.1% to about 10%, oralternatively from about 1% to about 8%. Mixtures of Group VIII metalscan also be used in conjunction with the SSZ-57 zeolite. Mixtures suchas Pt and Ni; Pt and Pd; Pd and Ni; and Pt, Pd, and Ni can be used innumerous different proportions to achieve a suitable catalyst.

The Group VIII metal may be added to the zeolite support by any suitablemethod known in the art. For example, the metal may be added to thezeolite support by ion-exchange or by impregnation. In general, themetals are added as salts, by filling the pores of the catalyst with asolution of appropriate concentration to achieve the desired metalloading. The metals may be added as salts, for example as salts ofthermally decomposable anions such as for example the nitrate, nitriteor, acetate salt. Alternatively, the metals may be added as solublemetal complexes. Following addition of the metal to the zeolite support,the mixture may be equilibrated, dried, and calcined to decompose thesalts or soluble complexes, remove solvent, remove impurities and/or toremove volatile products. Alternatively, adsorption or other techniqueswell known in the art for introducing metals into porous substances mayalso be used.

The xylene isomerization catalysts may be activated by reduction in thepresence of hydrogen and a base. The xylene isomerization catalysts maybe placed in a suitable reaction zone, for example a reactor vessel,where the catalyst may be subjected to a reducing atmosphere. Thereaction zone may be heated while carrying out the catalyst reduction.The catalyst may be optionally dehydrated in an inert gas such asnitrogen before reduction in the presence of hydrogen and the base. Inan embodiment, the catalyst is dehydrated in the presence of hydrogenand a base wherein hydrogen may be present in an amount equal to or lessthan about 100 vol. %. Additionally or alternatively, the reduction withhydrogen and the base may be carried out in the presence of nitrogen.Where nitrogen is present, the amount of hydrogen in the reducingatmosphere may range from about 1 to about 50 vol. %, alternatively fromabout 5 to about 35 vol. %, alternatively from about 5 to about 25 vol.%, alternatively from about 10 to about 25 vol. %, or alternatively fromabout 10 to about 20 vol. %, based upon the total volume of hydrogen andnitrogen present in the reaction zone. Thus, the reducing atmosphere maycomprise a base, hydrogen, and optionally an inert gas such as nitrogen.The amount of base introduced to the reaction zone may be a traceamount, for example an amount in a range of from about 1 ppmv to about10,000 ppmv, alternatively from about 5 ppmv to about 1000 ppmv, oralternatively, from about 10 ppmv to about 500 ppmv by total volume ofthe hydrogen and the inert gas.

In an embodiment, a base is fed into the reaction zone. Suitable basescan comprise ammonia, alkyl amines (R′R″R′″N; wherein R′,R″, and R′″ canbe alkyl radicals from 1 to 5 carbon atoms), hydrazine (H₂NNH₂), orcombinations thereof. In another embodiment, all or a portion of thebase may be introduced to the reaction zone by forming the base in situwithin the reaction zone. Nitrogen and hydrogen may be introduced to thereaction under reaction conditions sufficient to form the base, i.e.,ammonia. In an embodiment, the nitrogen and hydrogen may be introducedto the reaction in amounts and under conditions sufficient to form traceamounts of ammonia. Alternatively, the nitrogen and hydrogen may beintroduced to the reactor in the amounts set forth previously.

The reduction may be carried out at reaction conditions to reduce theGroup VIII metal, to form the base in situ within the reaction zone, orboth. In an embodiment, the reduction is performed at a temperature offrom about 500 to about 1500° F., alternatively from about 600 to about1200° F., alternatively from about 700 to about 950° F.; a pressure offrom about 0.5 to about 10 atm, alternatively from about 1 to about 5atm; a gas hourly space velocity (GHSV) of from about 10 to about 5000hr⁻¹, alternatively from about 500 to about 2000 hr⁻¹; and from about 1to about 100% hydrogen, alternatively from about 5 to about 25% hydrogenbased on the total volume of hydrogen and nitrogen present in thereaction zone.

The reduced xylene isomerization catalyst may be used in a xyleneisomerization/ethylbenzene hydrodealkylation process, which may besimply referred to as a “xylene isomerization process.” A xyleneisomerization unit typically serves at least two functions. First, itre-equilibrates the xylenes portion of a paraxylene-depleted stream byforming paraxylene from the xylene isomers orthoxylene and metaxylene.As used herein, “paraxylene-depleted stream” refers to a streamcontaining a below equilibrium level of paraxylene relative to the otherxylenes (orthoxylene and metaxylene) in the stream. Second, ittransalkylates or hydrodealkylates the ethylbenzene in theparaxylene-depleted stream to facilitate its removal from the C₈aromatics fraction. Since ethylbenzene boils in the same range as thexylene isomers, it is more economical to include it in the C₈ aromaticsfraction that is fed to the paraxylene separation process than attemptrecovery by distillation. As a result of the xylene isomerizationreaction, paraxylene is formed from the other components of theparaxylene-depleted stream. Moreover, due to the selectivity of thecatalyst for paraxylene, more than an equilibrium amount of paraxyleneis produced. Additional teaching regarding xylene isomerization may befound in U.S. Pat. Nos. 6,051,744 and 5,877,374, each of which isincorporated by reference herein in its entirety.

Ethylbenzene is normally not removed with the paraxylene in thecrystallization or adsorption step; moreover it is not efficientlyisomerized under the xylene isomerization conditions generally employed.Thus, it is highly desirable to remove as much ethylbenzene as possibleper pass by transalkylation or hydrodealkylation so that theethylbenzene does not accumulate in the recycle loop. If thisaccumulation were to occur, a bleed stream out of the paraxyleneproduction loop would be necessary to remove the ethylbenzene, whichwould reduce paraxylene production. Thus, a function of theisomerization unit is to react-out the ethylbenzene by simpledealkylation, hydrodealkylation, isomerization, ortransalkylation/disproportionation, depending on the type ofisomerization process. As noted previously, the conversion ofethylbenzene and the isomerization of orthoxylene and metaxylene arereferred to collectively herein as xylene isomerization.

A xylene isomerization catalyst which is effective in removal ofethylbenzene can to a much smaller extent catalyze side reactions thatlead to the destruction of the desired xylene product. There arenumerous side reactions leading to the destruction of xylene. Suchreactions include without limitation transalkylations anddisproportionation reactions such as shown in Equations 1-3:

Xylene+EB →Toluene+Methylethylbenzene  (1)

Xylene+EB→Benzene+Dimethylethylbenzene  (2)

Xylene+Xylene→Toluene+Trimethylbenzene  (3)

where EB represents ethylbenzene. The optimization of a xyleneisomerization catalyst may entail the maximization of EB conversionwhile simultaneously minimizing the loss of xylene via reactions such asthose given in Equations 1-3. In an embodiment, a xylene isomerizationcatalyst of this disclosure may function to maximize the overall xyleneselectivity by minimizing the loss of xylene. Herein, the xyleneselectivity may be may expressed as the mole ratio of ethylbenzeneconversion to the xylene loss (EB/XL). In an embodiment, the xyleneisomerization catalyst of this disclosure has an EB/XL of from about 70to about 600, alternatively from about 100 to about 450, alternativelyfrom about 150 to about 350.

Xylene isomerization may be performed by contacting the reduced xyleneisomerization catalyst with a paraxylene-depleted stream in anisomerization reaction zone under suitable reaction conditions such thatone or more components of the paraxylene-depleted stream undergo xyleneisomerization as described herein. In an embodiment, the isomerizationreaction zone is the same as the reduction reaction zone. For example,the xylene isomerization catalyst may be loaded into a reactor vessel,reduced in situ, and subsequently contacted with a paraxylene-depletedstream for isomerization thereof.

The paraxylene-depleted stream for the xylene isomerization process maybe obtained from a paraxylene separation unit comprising an adsorptionprocess, a crystallization process, or a combination thereof. Suchprocesses remove paraxylene from a C₈ aromatics fraction, therebyforming a paraxylene-depleted stream. The C₈ aromatics fraction may beobtained from reforming or aromatizing a wide boiling range naphtha in areformer, for example a continuous catalytic reformer (CCR) orsemi-regenerative reformer, which contains near equilibrium amounts oforthoxylene, metaxylene, and paraxylene, along with ethylbenzene. Inembodiments, the amount of paraxylene present in the paraxylene-depletedstream may be in a range of from about 0% to about 20%, alternativelyfrom about 0% to about 12%, by total weight of the stream. Aparaxylene-depleted stream that is obtained predominantly from aparaxylene separation unit that comprises an adsorption process willtypically have lower amounts of paraxylene than a paraxylene-depletedstream that is obtained from a paraxylene separation unit based on acrystallization process. The paraxylene-depleted stream may also have anethylbenzene concentration in a range of from about 5% to about 25%,alternatively from about 10% to about 20%, alternatively from about 9%to about 12%, by weight of the stream. The paraxylene-depleted streammay have a small concentration of non-aromatic compounds in a range offrom about 0% to about 15%, alternatively from about 0% to about 8%,alternatively from about 0% to about 5%, by weight of the stream. Thebalance of the paraxylene-depleted stream comprises mixed xylenes.

The xylene isomerization may be carried out at a relatively low pressureunder a relatively low flow of hydrogen, commonly referred to as a“trickle flow” of hydrogen. The trickle flow of hydrogen typicallypasses through the isomerization reaction zone in which the catalystresides once and thus no hydrogen recycle occurs. The hydrogen may befed to the isomerization reaction zone at a rate such that the moleratio of hydrogen to ethylbenzene ranges from about 1.0 to about 7.0,alternatively from about 1.0 to about 3.0. The hydrogen may be fed tothe isomerization reaction zone at a rate such that the ratio of molesof total hydrocarbon to moles of hydrogen ranges from about 0.01 toabout 1, alternatively from about 0.02 to about 0.45, alternatively fromabout 0.05 to about 0.25. The temperature in the isomerization reactionzone may range from about 500° F. to about 1000° F., alternatively fromabout 600° F. to about 900° F.

In an embodiment, the xylene isomerization is carried out using catalystcomprising a SSZ-57 support and Group VIII metal such as for exampleplatinum and this mixture is hereafter referred to as Pt/SSZ-57. In suchan embodiment, the Pt/SSZ-57 may function as a dual catalyst that iseffective for the isomerization of mixed xylenes and thehydrodealkylation of ethylbenzene. Such a catalyst may be pretreatedwith ammonia as has been described herein. In an embodiment, thePt/SSZ-57 is used as a catalyst in a trickle-hydrogen type process aspreviously described herein. In such an embodiment, the reaction may becarried out at a hydrogen pressure of less than about 50 psig and at aratio of about 1.2 mole hydrogen per mole of ethylbenzene in the feed.Such processes may be advantageous in reactors wherein hardwarelimitations prevent the operation of the reactor at elevated pressures.Alternatively, the Pt/SSZ-57 catalyst may be used in a xyleneisomerization process carried out at a high hydrogen pressure. In suchan embodiment, the hydrogen pressure may range from about 50 psig toabout 500 psig with about a 1-5:1 hydrogen/hydrocarbon ratio. Suchprocesses may be advantageous in reactors designed for high pressureoperation.

The xylene isomerization may be carried out in any suitable processequipment, for example a moving bed or fixed bed reactor. After reachingthe end of a reaction cycle in a moving bed reactor, the catalyst may beregenerated in a regeneration section/zone where coke is burned off ofthe catalyst in an oxygen-containing atmosphere such as air at arelatively high temperature. The catalyst may then be recycled to thereaction zone for further contact with the feed. In a fixed bed reactor,the catalyst may be regenerated by using an inert gas containing a smallamount of oxygen, e.g., from about 0.1 vol. % to about 2.0 vol. % bytotal volume of the gas, to burn the coke off of the catalyst in acontrolled manner so as not to exceed a maximum temperature of about950° F.

A reduced xylene isomerization catalyst as described herein may producea greater than equilibrium amount of paraxylene as demonstrated by aninitial PXATE (Paraxylene Approach to Equilibrium) of greater than about100%, wherein PXATE=100*{(wt % of total xylenes that arePX)_(product)−(wt % of total xylenes that are PX)_(feed)}/{(wt % oftotal xylenes that are PX)_(equilibrium)−(wt % of total xylenes that arePX)_(feed)}, where the term (wt % of total xylenes that arePX)_(equilibrium) is estimated from published temperature-dependantxylene equilibrium data, and is about 24 wt %. The catalyst alsotypically illustrates excellent stability from a paraxylene selectivitystandpoint as demonstrated by a very slow decline in the PXATE withtime. A slow decline in PXATE allows the cycle time between catalystregenerations to be increased and effectively decreases the overallaging rate of the catalyst.

A reduced xylene isomerization catalyst as described herein may exhibitexcellent stability from an ethylbenzene conversion standpoint. Aportion of the ethylbenzene in the paraxylene-depleted stream may beconverted by hydrodealkylation to, for example, benzene and ethane.Ethylbenzene conversion may be calculated as EBConv=100*(wt %EB_(feed)−wt % EB_(prod))/wt % EB_(feed). The EBConv may range fromabout 10 wt % to about 80 wt %, alternatively from about 20 wt % toabout 75 wt %, or alternatively from about 25 wt % to about 70 wt %.Stoichiometrically, one mole of hydrogen is required to hydrodealkylateone mole of ethylbenzene. However, in practice, more hydrogen isrequired since all of the hydrogen does not react and some of thehydrogen reacts with molecules other than ethylbenzene. For example, thehydrogen may be used in hydrogenating cracked paraffins in the reactionzone. The hydrogen may also saturate some of the aromatic rings. In anembodiment, about 1.2 moles of hydrogen are present for every mole ofethylbenzene.

The reduced xylene isomerization catalyst may be regenerated as needed.As a catalytic process continues over time, the catalyst activitygenerally decreases. To offset a decrease in activity, other processconditions may be adjusted to compensate for the decrease, for exampleby increasing reactor temperature. When the catalyst activity and/orprocess conditions reach a point where the process is no longerefficiently catalyzed, the catalyst may need to be regenerated, ifpossible. Regeneration of the reduced isomerization catalyst may beneeded when the ethylbenzene conversion becomes unacceptably low, whenthe non-aromatic conversion becomes unacceptably low, when the reactiontemperature becomes unacceptably high, or combinations thereof.

A reduced xylene isomerization catalyst as described herein may reducethe xylene loss that occurs during the isomerization. As used herein,xylene loss (“XylLoss”) refers to the percentage of the xylene isomerslost to hydrodealkylation and other processes, and is calculated by theequation, wherein XylLoss=100*[(total wt % xylenes)_(feed)−(total wt %xylenes)_(product)]/(total wt % xylenes)_(feed). The XylLoss during acontinuous xylene isomerization run over a period of about 24 hours mayrange from about 0.0 wt % to about 5.0 wt %, alternatively from about0.1 wt % to about 3.0 wt %, alternatively from about 0.2 wt % to about2.0 wt %, alternatively from about 0.2 wt % to about 1.0 wt %, oralternatively from about 0.8 wt % to about 1.2 wt % by weight of thexylenes when the ethylbenzene conversion is about 50 wt %. Further, axylene loss during a continuous run of the isomerization of the xylenesover a period of about 4 weeks may be in a range of from about 0.3 wt %to about 0.5 wt % by weight of the xylenes when the ethylbenzeneconversion is about 50 wt %.

Without intending to be limited by theory, it is believed that theaddition of the base during reduction of the xylene isomerizationcatalyst serves to hinder the reaction rate and number of unwanted sidereactions that would otherwise occur during the isomerization. Inparticular, the base may adsorb on the acid sites of the zeolite whereit typically bonds with the more reactive, stronger acid sites anddesorbs from the weaker acid sites. As such, the base may selectivelypassivate a portion of the acid sites while leaving the other acid sitesintact. It is believed that the stronger acid sites aredisproportionately passivated. Those passivated acid sites are no longerfree to react with the xylene isomers, and thus a reduced xylene loss isobserved at a constant ethylbenzene conversion.

EXAMPLES

The invention having been generally described, the following examplesare given as particular embodiments of the invention and to demonstratethe practice and advantages thereof. It is understood that the examplesare given by way of illustration and are not intended to limit thespecification or the claims to follow in any manner. In the followingexamples, the data is presented for the first 1000 hours on stream (HOS)of the isomerization run.

Comparative Example 1

Xylene isomerization was carried out in a bench scale reactor using axylene isomerization catalyst comprising 0.25 weight percent platinum onan HZSM-5 zeolite support. The HZSM-5 zeolite support further comprised80 weight percent zeolite and 20 weight percent binder. Theparaxylene-depleted stream comprised, in weight percent, from about 3 toabout 10 wt % non-aromatic compounds, from about 8 to about 12 wt %ethylbenzene, from about 3 to about 5 wt % paraxylene, from about 51 toabout 55 wt % metaxylene, and from about 23 to about 24 wt %orthoxylene. A 0.5-inch diameter stainless steel reactor vessel having alength of about 45 inches was loaded with 53 grams of the catalyst,resulting in a catalyst volume of about 73 cc and a catalyst bed heightof about 7 inches. The catalyst was dried in the reactor by feedingnitrogen to the reactor at 1.4 L/min and 25 psig. The reactortemperature was ramped at 25° F./hr from ambient temperatures to 500° F.and held at 500° F. until the water content of the reactor effluentdropped below 100 ppm. After drying, the catalyst was reduced byreplacing the nitrogen flow to the reactor with hydrogen at 1.4 L/minand 75 psig. The reduction temperature was ramped from 500 to 800° F. at10° F./hr, and held at 800° F. for fourteen hours before being allowedto cool to 625° F. They hydrogen flow rate was then reduced to 76 cc/minand the reactor pressure was reduced to 35 psig. The paraxylene-depletedstream (dried using 4A molecular sieves) was then fed to the reactor at225 gm/hr (260 cc/hr). The reactor was then operated at 625° F., 35psig, and a weight hourly space velocity (WHSV) of 4.2 hr⁻¹ for 1 hour.The temperature was then ramped from 625° F. at 1° F./hr until 50 wt %ethylbenzene conversion was achieved, which normally occurred at about640° F. The reactor effluent was sampled and tested via gaschromatograph, and the results are provided in Table 4 below.

Comparative Example 2

The catalyst used in Comparative Example 1 was regenerated in situ byflowing nitrogen at about 1200 cc/min mixed with enough air to providean oxygen concentration of about 0.5 vol. % based on a total gas flow ofabout 1230-1240 cc/min, to the reactor at 70 psig and 500° F. After twohours the water content of the reactor effluent had dropped below 100ppm, the reactor temperature was then ramped at 10° F./hr to 700° F. andheld at 700° F. for 24 hours. Subsequently, the oxygen content wasincreased to 1.0 vol. % and the reactor was held at 700° F. for another24 hours. Thereafter, the air flow was stopped, nitrogen was increasedto 1.4 L/min, and the reactor was allowed to cool to 500° F.Subsequently, the regenerated catalyst was reduced and aparaxylene-depleted stream was introduced as described in ComparativeExample 1. The results of the tests performed in this example are alsoshown in Table 4 below.

Example 1

The same procedure followed in Comparative Example 1 was performed withthe exception that the dried catalyst was reduced in the presencehydrogen, nitrogen, and a base, i.e., ammonia. More specifically, amixture of 20 vol. % hydrogen and 80 vol. % nitrogen was flowed thoughammonia permeation tube (Commercially available from Valco InstrumentsCo.) and fed to the reactor at 1.4 L/min and 75 psig, resulting in about100 ppmv ammonia in the reduction gas. The hydrogen flow rate wasadjusted, depending on the [EB] to maintain a 1:2 ratio ofhydrogen:hydrocarbon. The reduction temperature was ramped from 500° F.to 800° F. at 10° F./hr, held at 800° F. for fourteen hours, and allowedto cool to 625° F. The xylene isomerization results of the testsperformed in this example are presented in Table 4 below.

TABLE 4 Comparative Comparative Example 1 Example 2 Example 1 H₂/EB MoleRatio 1.20 1.20 1.20 Concentration of 8-10 11-12 11-12 EB in Feed, wt. %EB Conversion, % 50.9 49.3 50.9 Xylene Loss, % 0.92 1.17 ~0.2 EB/XL 65.644.4 200-300 PXATE, % 101.6 101.1 100.8

As illustrated in Table 4, the xylene loss that occurred during thexylene isomerization process was surprisingly much lower when thecatalyst was reduced in the presence of both ammonia and hydrogen(Example 1) rather than in the presence of only hydrogen (ComparativeExamples 1 and 2). The PXATE achieved by the catalyst reduced in theammonia and the hydrogen was nearly equivalent to that achieved by theconventionally reduced catalysts. The results also demonstrate that thecatalyst reduced in the presence of ammonia and hydrogen (Example 1) hada significantly higher xylene selectivity as indicated by the EB/XLratio.

While preferred embodiments of the invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the spirit and teachings of the invention. Theembodiments described herein are exemplary only, and are not intended tobe limiting. Many variations and modifications of the inventiondisclosed herein are possible and are within the scope of the invention.Where numerical ranges or limitations are expressly stated, such expressranges or limitations should be understood to include iterative rangesor limitations of like magnitude falling within the expressly statedranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4,etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). Use of theterm “optionally” with respect to any element of a claim is intended tomean that the subject element is required, or alternatively, is notrequired. Both alternatives are intended to be within the scope of theclaim. Use of broader terms such as comprises, includes, having, etc.should be understood to provide support for narrower terms such asconsisting of, consisting essentially of, comprised substantially of,etc. The various embodiments and components thereof disclosed herein maybe used singularly or in combination with any other embodiment disclosedherein.

Accordingly, the scope of protection is not limited by the descriptionset out above but is only limited by the claims which follow, that scopeincluding all equivalents of the subject matter of the claims. Each andevery claim is incorporated into the specification as an embodiment ofthe present invention. Thus, the claims are a further description andare an addition to the preferred embodiments of the present invention.The discussion of a reference herein is not an admission that it isprior art to the present invention, especially any reference that mayhave a publication date after the priority date of this application. Thedisclosures of all patents, patent applications, and publications citedherein are hereby incorporated by reference, to the extent that theyprovide exemplary, procedural or other details supplementary to thoseset forth herein.

1. A method of isomerizing xylenes, comprising: reducing a catalystcomprising a HZSM-5 zeolite support, a SSZ-57 zeolite support orcombinations thereof and a Group VIII metal in the presence of a gascomprising a base and hydrogen to produce a reduced catalyst; andcontacting the reduced catalyst with a paraxylene-depleted streamcomprising metaxylene and orthoxylene under reaction conditions suchthat portions of the metaxylene and orthoxylene are isomerized toparaxylene.
 2. The method of claim 1, wherein the base is present in arange of from about 1 ppmv to 10,000 ppmv by total volume of thehydrogen and an inert gas.
 3. The method of claim 1, wherein the basecomprises ammonia, alkyl amines, hydrazine, or combinations thereof. 4.The method of claim 1, wherein the base comprises ammonia formed in situfrom nitrogen and hydrogen in the presence of the reduced catalyst. 5.The method of claim 1, wherein the gas comprises hydrogen, the base, andnitrogen.
 6. The method of claim 1, wherein the Group VIII metalcomprises platinum, palladium, nickel, or combinations thereof.
 7. Themethod of claim 1 wherein the paraxylene-depleted stream comprises astream from a paraxylene separation unit.
 8. The method of claim 7wherein the paraxylene-depleted stream is formed via separation ofparaxylene from a C₈ aromatics fraction from a naphtha reformereffluent.
 9. The method of claim 1, wherein the paraxylene-depletedstream further comprises ethylbenzene and a portion of the ethylbenzeneis converted to other compounds.
 10. The method of claim 9 wherein theethylbenzene is converted to benzene via hydrodealkylation.
 11. Themethod of claim 9 wherein from about 10% to about 80% by weight of theethylbenzene is converted to other compounds.
 12. The method of claim 1,wherein an average xylene loss during isomerization of theparaxylene-depleted stream over a continuous period of about 24 hours isin a range of from about 0.0 wt % to about 5.0 wt % by weight of thexylenes when ethylbenzene conversion is about 50 wt %.
 13. The method ofclaim 1 wherein a mole ratio of ethylbenzene conversion to xylene lossis from about 70 to about
 600. 14. The method of claim 1 having aninitial PXATE about equal to or greater than 100%.
 15. A method ofreducing xylene loss during isomerization of a paraxylene-depletedstream, comprising passivating a portion of acid sites on a zeoliteisomerization catalyst by reducing the zeolite isomerization catalyst inthe presence of a gas comprising a base and hydrogen.
 16. The method ofclaim 15, wherein the zeolite isomerization catalyst comprises a HZSM-5zeolite support, a SSZ-57 zeolite support or combinations thereof and aGroup VIII metal.
 17. The method of claim 15, wherein the base comprisesammonia, alkyl amines, hydrazine, or combinations thereof.
 18. Themethod of claim 15, wherein the base comprises ammonia formed in situfrom nitrogen and hydrogen in the presence of the zeolite isomerizationcatalyst.
 19. The method of claim 15, wherein the paraxylene-depletedstream comprises ethylbenzene and from about 10% to about 80% by weightof the ethylbenzene is converted to other compounds.
 20. The method ofclaim 15, wherein an average xylene loss during isomerization of theparaxylene-depleted stream over a continuous period of about 24 hours isin a range of from about 0.0% to about 5.0% by weight of the xyleneswhen ethylbenzene conversion is about 50%.
 21. The method of claim 15having an initial PXATE about equal to or greater than 100%.
 22. Themethod of claim 15, wherein the gas comprises hydrogen, the base, andnitrogen.
 23. The method of claim 15 wherein a ratio of ethylbenzeneconversion to xylene loss is from about 70 to about 660.